The ability to amplify ribonucleic acid (RNA) is an important aspect of efforts to elucidate biological processes. To date, RNA (generally, mRNA) amplification is most commonly performed using the reverse transcriptase-polymerase chain reaction (RT-PCR) method and variations thereof. These methods are based on replication of RNA by reverse transcriptase to form single stranded DNA complementary to the RNA (cDNA), which is followed by polymerase chain reaction (PCR) amplification to produce multiple copies of double stranded DNA. Although these methods are most commonly used, they have some significant drawbacks: a) the reactions require thermocycling; b) the products are double stranded, thus rendering them less accessible to binding to probes; c) the reactions are prone to contamination with products of prior amplification, thus requiring strict containment of reaction mixtures; and d) the exponential nature of amplification of these methods renders them prone to generate pools of products which do not truly reflect the representation of the various RNA sequences in the input total RNA sample, due to unequal efficiency of amplification of different sequences, and the nature of exponential amplification which is based on replication of amplification products rather than on continued replication of the input target RNAs.
The total cellular mRNA represents gene expression activity at a defined time. Gene expression is affected by cell cycle progression, developmental regulation, response to internal and external stimuli and the like. The profile of expressed genes for any cell type in an organism reflects normal or disease states, response to various stimuli, developmental stages, cell differentiation, and the like.
Various methods for the analysis of gene expression have be developed in recent years. See, for example, U.S. Pat. Nos. 5,744,308; 6,143,495; 5,824,517; 5,829,547; 5,888,779; 5,545,522; 5,716,785; 5,409,818; EP 0971039A2; EP0878553A2. These include quantification of specific mRNAs, and the simultaneous quantification of a large number of mRNAs, as well as the detection and quantification of patterns of expression of known and unknown genes. The analysis of gene expression profiles is currently one of the most powerful tools in the study of cellular differentiation and cellular development, and in the investigation of normal and disease states of various organisms, in particular in human. This analysis is crucial for gene discovery, molecular medicine and drug discovery processes.
Essential for gene expression profiling is the ability to randomly amplify the total cellular mRNAs prepared from any cell or tissue. Although analysis of non-amplified mRNA is feasible, a significant amount of starting mRNA would be required. However, the total amount of sample mRNA that is available is frequently limited by the amount of biological sample from which it is derived. Biological samples are often limited in amount and precious. Moreover, the amount of the various mRNA species is not equal; some species are more abundant than others, and these are more likely and easier, to analyze. The ability to amplify mRNA sequences enables the analysis of less abundant, rare mRNA species. The ability to analyze small samples, by means of nucleic acid amplification, is also advantageous for design parameters of large scale screening of effector molecule libraries, for which reduction in sample volume is a major concern both for the ability to perform very large scale screening or ultra high throughput screening, and in view of the limiting amounts of library components.
Therefore, there is a need for improved RNA amplification methods that overcome drawbacks in existing methods. The invention provided herein fulfills this need and provides additional benefits.
All references cited herein, including patent applications and publications, are incorporated by reference in their entirety.